Electro-erosion machining of metal parts by electric discharge cutting or contouring is generally well-known. The process calls for juxtaposing a traveling, continuous wire-type electrode and a moving table or pair of tables on which a metal workpiece is mounted. A start hole is pre-drilled in the workpiece for threading the wire electrode. The wire travels from a supply reel so that new electrode wire surfaces are continually introduced to the erosion process. The table or tables move to continually position a metal workpiece, which is mounted on the table or tables, relative to the wire according to the progress of the process and the desired geometry for the finished part to be machined out of the workpiece, whereby electrical energy taking the form of successive electrical discharges between the moving wire and the workpiece is applied through a machining fluid constituted by a liquid dielectric. The electric energy removes material from the workpiece as the table or tables continually position the workpiece relative to the axially moving wire.
Conventionally, the workpiece is mounted or clamped to a work block on the work table or work tables of the standard wire EDM machine. Typically, programmable drives move the table or tables and workpiece along orthogonal axes, labeled "the X axis" and "the Y axis", to generate a machining path conforming to the desired contour shape. It is to be appreciated that the table or tables are capable of performing for both straight line motions and continuous path contouring. The workpieces are machined using power settings to achieve high metal removal rates. High speed machining, however, sacrifices both accuracy and surface finish.
Certain parts, for example, circular cams that are machined out of large plate or block workpieces, must be machined for highly accurate contours and exact surface finishes. Such precision machining is accomplished by re-machining the cut parts or "skim cutting" them several times to achieve a desired finished. But to skim cut, the workpiece must be held on the work table fixture or fixtures in its original position for the several passes. Where, as in some machines, the workpiece is clamped between two tables, the workpiece falls free when it is initially cut, virtually foreclosing the possibility of relocating and mounting the workpiece for secondary skim cutting.
To avoid this difficulty, it is a common practice to program the machining path so that parts of the workpiece will be left connected to the primary work block to form support bridges. Subsequent remachining or "skim cutting" can then be performed without the workpiece moving in relationship to the table axes. The connecting links may subsequently be cut with excess stock left on the workpiece to allow for finishing the bridge surface close to the "skim cut" part surface. This procedure, however, requires additional start holes to be pre-drilled within the workpiece to allow for rethreading the wire electrode on opposite sides of the support bridge.
Even avoiding this means of supporting a workpiece does not avoid the certain difficulties associated with controlling a workpiece along rectangular coordinates to approximate a circular path for cutting and contouring. Practice has revealed that complex components must be incorporated in programs for complex curves in the contours of workpieces. To simplify the programs, and thereby reduce the expense of man-hours needed to develop more complex programs, simple curves of the workpiece contours have been used to describe the path of motion between the electrodes and the workpieces. Simple curves, including circles, ellipses, parabolas, and the like, are themselves first order, straight-line approximations of circles, ellipses, parabolas, and the like, which then must be painfully combined into workpiece contours as second order approximations of the curves of the workpiece contours.